The present invention relates to a dry flue gas desulfurization process also called a spray dry flue gas desulfurization process, incorporating nitrogen oxides removal.
The major contributors to acid rain are sulfur oxides, SO.sub.x, and nitrogen oxides, NO.sub.x. Sulfur oxides (SO.sub.x) as an air pollutant, include sulfur dioxide (SO.sub.2), sulfur trioxide (SO.sub.3), plus the corresponding acids (H.sub.2 SO.sub.3 and H.sub.2 SO.sub.4) and salts (sulfites and sulfates). Nitrogen oxides (NO.sub.x) in air pollution terminology refer to two gaseous oxides of nitrogen, nitrogen dioxide (NO.sub.2) and nitric oxide (NO). The predominant species in flue gas are SO.sub.2 and NO. SO.sub.x can generally be controlled reasonably well by flue gas desulfurization processes. Dry Flue Gas Desulfurization (DFGD), also called Spray Dry Flue Gas Desulfurization (SDFGD), is a sulfur dioxide scrubbing process in which lime slurries, or alternatively, sorbent solutions containing sodium compounds such as sodium bicarbonate, are contacted with flue gas in a device such as a spray dryer, in which the sulfur dioxide reacts with the sorbent material, and the water simultaneously evaporates. The resulting product is a dry mixture of CaSO.sub.4, CaSO.sub.3 unreacted sorbent, and flyash. A particulate removal device such as a baghouse located downstream from the spray absorber removes entrained particulates which are not removed in the absorber vessel.
It is expected that acid rain regulation will require the scrubbing of nitrogen oxides as well as sulfur oxides. Conventionally operated SDGFD systems do not remove NO.sub.x except for perhaps a small percentage in the baghouse. NO.sub.x control is not easily accomplished without the addition of expensive processes such as selective catalytic reduction. Proposed targets for stack gas cleanup are 90% SO.sub.x / 90% NO.sub.x and 50% SO.sub.x / 50% NO.sub.x for new and retrofit applications, respectively.
Niro Atomizer in a paper by Felsvang et al., describes an approach for operating SDFGD processes in which the absorber exit temperature is increased by about 50.degree. F., so that the temperature entering the baghouse is about 195.degree.-215.degree. F., rather than the normal 150.degree.-160.degree. F. This results in NO.sub.x removal in the baghouse, but other undesirable process alterations are necessary. In order to maintain high SO.sub.2 removal, it is necessary to increase the temperature entering the absorber, which is undesirable from a plant efficiency standpoint. Sodium compounds such as NaOH are added to the sorbent to increase its scrubbing effectiveness, but this is undesirable due to the high cost of NaOH and the fact that the wastes will be leachable resulting in additional land fill disposal expense.
It is an object of the present invention to provide a low cost SO.sub.x /NO.sub.x scrubbing system.
It is a further object of the present invention to provide a method for removing NO.sub.x from flue gas streams in spray dry flue gas desulfurization type processes.